1. Field of the Invention
The present invention generally relates to positional error detection methods and apparatuses and computer-readable storage media, and more particularly to a positional error detection (or color registration error detection) method and a positional error detection apparatus which detects an amount of positional error among images of a plurality of colors when overlapping the images of the plurality of colors to obtain a visible image in a tandem type image forming apparatus. The present invention also relates to a computer-readable storage medium which stores a program for causing a computer to execute such a positional error detection method.
2. Description of the Related Art
In the so-called tandem type image forming apparatus, an image forming means is provided for each of four colors, and toner images formed by the four image forming means are overlapped directly on a recording medium such as paper or, on an intermediate transfer belt, in order to form a color image.
In such a tandem type image forming apparatus, the overlapping positions of the images of the four colors slightly deviate from one another, to thereby prevent a stable color image from being formed. For this reason, a positional error (or color registration error) correction is carried out in general, as proposed in a Japanese Laid-Open Patent Application No. 2003-266698, for example. According to the positional error correction, a positional error correction mark is formed in each of the four colors, and image positions of the four colors are detected by a detecting means in order to match the overlapping positions of the four colors. In general, detection results of the color marks in cyan, magenta and yellow are compared with a detection result of the reference color mark in black, and the amount of positional error is computed for each color mark with respect to the reference color mark.
The reflected light detected by the detecting means includes a regular (or mirror) reflection light component and a diffused reflection light component. The regular reflection light that has been strongly reflected by a transport belt is absorbed by the toner image. On the other hand, the diffused reflection light that has been weakly reflected by the toner image of the color mark is absorbed by the transport belt and the black toner image.
Accordingly, when detecting the toner images of the color marks using the conventional detection method, the toner images are detected from the reflected light in which the regular reflection light component and the diffused reflection light component coexist. However, if the alignment of a light emitting element and a light receiving element of the detecting means with respect to an external or internal function of the apparatus is off or, the adjustment of the positional relationship of the light emitting element and the light receiving element are off due to mechanical tolerance, assembling error or the like, a peak position of the regular reflection light component and a peak position of the diffused reflection light component will not match. Consequently, there is a problem in that an error is introduced in the detected positions of the color marks.